Backlights are used to illuminate liquid crystal displays (LCDs). LCDs with backlights are used in small displays for cell phones and personal digital assistants (PDA), as well as in large displays for computer monitors and televisions. Typically, the light source for the backlight includes one or more cold cathode fluorescent lamps (CCFLs). The light source for the backlight can also be an incandescent light bulb, an electroluminescent panel (ELP), or one or more hot cathode fluorescent lamps (HCFLs).
The display industry is enthusiastically pursuing the use of LEDs as the light source in the backlight technology because CCFLs have many shortcomings: they do not easily ignite in cold temperatures, require adequate idle time to ignite, and require delicate handling. LEDs generally have a higher ratio of light generated to power consumed than the other backlight sources. So, displays with LED backlights consume less power than other displays. LED backlighting has traditionally been used in small, inexpensive LCD panels. However, LED backlighting is becoming more common in large displays such as those used for computers and televisions. In large displays, multiple LEDs are required to provide adequate backlight for the LCD display.
The number of LEDs required for a given display, and the cost to manufacture the display, can be reduced by increasing the amount of light produced by each LED. The amount of light produced by an LED, or luminous intensity, is a function of the current in the LED. As shown in FIG. 1, the luminous intensity of an LED increases with increasing current in the LED. However, there is a limit to how high the intensity of an LED can reliably be increased by increasing the current. This limit is shown as IMAX in FIG. 1. IMAX is generally expressed as the mean operating current. The current may be continuous or discrete, in which case IMAX is the average current calculated by the product of the delta (or difference) between maximum and minimum current and the duty cycle. At currents near or above IMAX, there is a high probability that the LED will catastrophically fail. Operating LEDs at such conditions leads to reliability problems in displays and higher repair and warranty costs for display manufacturers. Therefore, display manufacturers generally do not drive LEDs at or above IMAX.
One of the challenges facing display manufactures is that IMAX is not constant. As shown in FIG. 2, IMAX 20 is a function of the temperature of the medium surrounding the LEDs, or LED ambient temperature. FIG. 2 shows that IMAX is nearly constant over an ambient temperature range up to the slope transition temperature, TSLP 21. Once the ambient temperature reaches TSLP, IMAX decreases with increasing ambient temperature until the ambient temperature reaches TMAX. When the ambient temperature reaches TMAX 23, no current can be applied to the LED without a high risk of catastrophic failure. LED manufactures often provide customers with TMAX curves like that in FIG. 2 so that display manufactures can avoid conditions that result in a high probability of LED failure. LED manufactures generally recommend that the LEDs operate in the range below the TMAX curve, the safe operating area.
The LED ambient temperature is largely a function of the environment in which the display is placed. Many display applications, such as in automobiles, are subject to high temperatures and large temperature fluctuations. Therefore, display manufactures are faced with a tradeoff between competing options. Display manufactures may run LEDs at a lower current that is within the safe operating area over a larger temperature range. But this requires more LEDs per display for a given intensity. Or display manufactures can choose to run the LEDs at a higher current but face reliability issues at higher ambient temperatures.
One approach to maintaining LED current below IMAX is to control the LED ambient temperature. If the LED ambient temperature is controlled to less than TSLP, then the LED current can safely be maintained constant at or near the maximum value of IMAX. This approach has the benefits of allowing the LEDs to run at the maximum safe current and not requiring changes to the current in the LEDs based on changes in the ambient temperature. However, regulating temperature generally requires additional devices to be added to the display. The additional temperature-regulating devices are expensive to manufacture, expensive to operate, bulky and noisy. Because of these limitations, temperature-regulating devices are not generally used in displays to control the LED ambient temperature. Even when temperature-regulating devices, such as heat sinks, are used to control the LED ambient temperature, they may not provide sufficient temperature control to allow the LED current to operate at or near IMAX.
Another approach is to maintain the LED current at a value below ISAF 22 at all times, as shown in FIG. 2. At currents below ISAF, LEDs have the largest possible safe ambient temperature range. A benefit of this approach is simplicity. An exemplary circuit for maintaining the LED current below ISAF is shown in FIG. 3. In this circuit, the value of the resistor RSET 31 can be determined from values of the input voltage (VSET 32), the forward voltage (VF) of the LEDs 33, and the maximum allowed current ISAF. A disadvantage of this approach is that the LEDs 33 are not utilized to their maximum potential. At all LED ambient temperatures below TMAX, the current in the LEDs 33 cannot be increased to go outside the safe operating area. Therefore, for a given intensity requirement of a display, more LEDs might be required.
Another approach is to use a negative temperature coefficient resistor and logic to control the current in the LEDs. An example of this approach is shown in FIG. 4. The negative temperature coefficient resistor, RNTC 41, is located so as to be at the same ambient temperature as the LEDs 43. As the LED ambient temperature increases, the resistance of RNTC decreases. HCxThe input voltage, VL 42, is held relatively constant and is independent of the LED ambient temperature. As the resistance of RNTC decreases, the voltage, VN 44, decreases. The logic 40 compares VN to a constant reference set point voltage, VS 45. In one embodiment, the logic 40 is a three-input operational amplifier. When VN is greater than VS, the logic drives the current in the LEDs to VS/RSET. When VN is less than VS, the logic 40 drives the current in the LEDs to VN/RSET. As shown in FIG. 5, the voltages and components of the above circuit are designed so that current in the LEDs is at or near IMAX for all temperatures below TSLP 53. The current curve given by VS/RSET and the current curve given by VN/RSET 52 intersects at or near TSLP 53. A disadvantage of this solution is that it requires the use of an expensive negative temperature coefficient resistor 41. Further, the negative temperature coefficient resistor 41 of the above circuit cannot readily be made part of the same integrated circuit as the logic 40.
The present invention solves these problems and provides an ambient temperature-based current controller for LEDs that is inexpensive and manufacturable as a single integrated circuit or on multiple integrated circuit chips.